In these days, a thermal reactor is largely utilized to incinerate or pyrolyze municipal refuse for disposal although there are other processes available for disposing of the refuse.
For such process, a reactor of a fluidized bed type has been recently employed rather than a stoker since the fluidized bed type reactor is superior in the incinerating efficiency and the amount of residue produced thereby is relatively small.
If the reactor having the fluidized bed is utilized the materials are effectively thermally processed in the beds almost instantaneously. There is substantially no moving elements in the portions exposed to the high temperature in the fluidized bed reactor and, thus, there is little risk that these portions will be damaged. For the above and other reasons, the fluidized bed type reactors have been generally considered desirable for disposal of refuse.
On the other hand, to maintain proper working conditions for smooth operation of the fluidized bed attention must be paid to several factors.
For instance, if the size of the materials to be charged into the bed varies, the fluidizing condition may be affected. That is, if the size of the refuse is beyond a certain size, it may obstruct fluidization of the fluidizing medium and therefore, the refuse has usually been shredded or crushed before being charged into the fluidized bed. Therefore, a shredder or crusher is required, thereby increasing the equipment cost and creating the necessity of space for installing such shredder or crusher. Also, the refuse generally contains several different kinds of materials including non-combustible materials such as metallic items, concrete blocks etc. or large combustible items which are not easily shredded such as bed clothes, blankets and fishing nets, etc. These items may cause abrasion or fracture of the shredding or crushing edges of the shredder or crusher and may impose an overload which stops the shredder or crusher whereby the maintenance cost is increased and the operational efficiency is decreased.
Also, in the fluidized bed, the charged material is instantaneously incinerated due to the fluidization and, thus, where a large quantity of the material is charged thereinto at one time, such material tends to burn spontaneously thereby temporarily causing imperfect combustion and generation of black smoke.
Therefore, in addition to the shredding operation explained above, the feed rate of the material to the fluidized bed has been preferably kept constant.
In most of the fluidized bed type reactors, the main problems have resulted from failures in the shredder or crusher or in the feeder for feeding the shredded material at a uniform rate rather than from failures in the body of the reactor chamber itself.
Another problem encountered in the fluidized bed type incineration is that the materials, particularly organic materials, may be incinerated at the top of the fluidized bed and, thus, the generated heat produced by such incineration may not be recovered in most cases and is merely discharged through the free board of the fluidized bed. Also, in case of generation of pyrolysis gas, it is also usually consumed without the heat therefrom being utilized. Therefore, even when the incineration of the materials or combustion of the generated pyrolysis gas proceeds, the thermal energy derived therefrom has been wasted in most cases and, thus, the temperature of the fluidizing medium or sand is gradually lowered during the process. Therefore, under such conditions, auxiliary burning means is sometimes required to maintain the temperature of the sand at a certain level to continue the incineration. In order to utilize such thermal energy which heretofore was wasted and to obviate or reduce the need for an auxiliary burning means, the inside of the fluidized bed has been divided by a partition wall to produce a downwardly moving bed portion and a fluidized bed portion, the two portions being in communication with each other at the top portions and the bottom portions respectively so that the fluidizing medium circulates between the two portions in such a way that the medium or sand descends together with the combustible items in the moving bed portion and is fed to the fluidized bed portion at the bottom and the sand and the combustible items are fluidized and incinerated within the fluidized bed portion while they are moved upwardly and then they are fed transversely again to the top of the downwardly moving bed portion. With this arrangement, the thermal energy generated during the incineration is properly recovered by the medium or sand. Such an arrangement is disclosed, for example, in UK Pat. No. 1577717.
However, with this arrangement too, there are several drawbacks as noted below.
a. The items having a large size may clog the gap between the lower end of the partition wall and the diffusion plate and this may disturb smooth circulation of the fluidizing medium, lower the combustion efficiency and/or prevent the non-combustible items from being smoothly discharged.
b. Descending rate of the moving bed portion is small and the amount of circulating fluidizing medium is limited whereby the capacity for incineration is limited.
c. The range within which the descending rate of the moving bed can be controlled is small.
d. The pyrolysis gas generated during the descend is prevented from being delivered to the combustion zone, i.e. the fluidized bed whereby such gas is merely burnt in the free board without being used for heating the medium.
e. Since the fluidized bed portion generally surrounds the moving bed portion, the feeding duct for the refuse transverses the portion above the fluidized bed portion whereby the duct is subjected to abrasion, corrosion and/or deformation which may result in clogging of the duct.
f. Because of the presence of the partition wall and/or duct within the space of the thermal reactor, inspection or repair of the inside of the thermal reactor is obstructed and inconvenient.
Accordingly, it has been desired to have a method capable of using a thermal reactor or incinerator without the drawbacks described above.